Im wondering what to go with... The HD 6870x2, HD 7850, or the 7870?
Is there much difference between the 7850 and the 7870?
6870x2 should be better or the same than a 7850. Go for the single card solution since it's less of a hassle. From what I read, it's driver hell if you plan on running it on crossfire and also it'll cost you more.
The difference between the 7850 and the 7870 is 100 bucks so it's pretty much a matter of how much money you're willing to spend for what kind of performance.